In this work, we investigate the mode of binding of all nine hydrogen-bonded structures of the adenine...thymine base pair. The planar H-bonded structures were optimized at the MP2/cc-pVTZ level, and the respective interaction energies, corrected for the basis set superposition error, were determined with the aug-cc-pVDZ basis set. The energy components were obtained from the DFT-SAPT procedure using the aug-cc-pVDZ basis set. The charge-transfer character of the single structures was estimated using NBO characteristics. It was established that dipole-dipole interaction itself cannot explain the preferred structure of the pair. Of the various energy components, first-order electrostatic energy plays the most important role. Second-order energy (the sum of induction and dispersion energies) amounts to about 56% of the electrostatic energy. The delta(HF) term covering among others the charge-transfer energy is rather large. The importance of delta(HF) is reflected by the NBO characteristics and especially by the NBO charge-transfer energy. The sum of the second-order energy and the delta(HF) term is only slightly smaller than the electrostatic energy (75-77%), which reflects the importance of the nonelectrostatic terms even in the case of strong H-bonded complexes. The WC structure, which exists in DNA, represents the seventh local minimum, while the three most stable structures utilize the N9-H proton donor group of the five-membered ring.
- MeSH
- Adenine chemistry MeSH
- Algorithms MeSH
- Biophysics methods MeSH
- DNA chemistry MeSH
- Financing, Organized MeSH
- Nucleic Acid Conformation MeSH
- Molecular Conformation MeSH
- Models, Molecular MeSH
- Base Pairing MeSH
- Energy Transfer MeSH
- Spectrophotometry, Infrared methods MeSH
- Static Electricity MeSH
- Thymine chemistry MeSH
- Hydrogen Bonding MeSH
The CCSD(T) interaction energies for the H-bonded and stacked structures of the uracil dimer are determined at the aug-cc-pVDZ and aug-cc-pVTZ levels. On the basis of these calculations we can construct the CCSD(T) interaction energies at the complete basis set (CBS) limit. The most accurate energies, based either on direct extrapolation of the CCSD(T) correlation energies obtained with the aug-cc-pVDZ and aug-cc-pVTZ basis sets or on the sum of extrapolated MP2 interaction energies (from aug-cc-pVTZ and aug-cc-pVQZ basis sets) and extrapolated DeltaCCSD(T) correction terms [difference between CCSD(T) and MP2 interaction energies] differ only slightly, which demonstrates the reliability and robustness of both techniques. The latter values, which represent new standards for the H-bonding and stacking structures of the uracil dimer, differ from the previously published data for the S22 set by a small amount. This suggests that interaction energies of the S22 set are generated with chemical accuracy. The most accurate CCSD(T)/CBS interaction energies are compared with interaction energies obtained from various computational procedures, namely the SCS-MP2 (SCS: spin-component-scaled), SCS(MI)-MP2 (MI: molecular interaction), MP3, dispersion-augmented DFT (DFT-D), M06-2X, and DFT-SAPT (SAPT: symmetry-adapted perturbation theory) methods. Among these techniques, the best results are obtained with the SCS(MI)-MP2 method. Remarkably good binding energies are also obtained with the DFT-SAPT method. Both DFT techniques tested yield similarly good interaction energies. The large magnitude of the stacking energy for the uracil dimer, compared to that of the benzene dimer, is explained by attractive electrostatic interactions present in the stacked uracil dimer. These interactions force both subsystems to approach each other and the dispersion energy benefits from a shorter intersystem separation.
The free-energy surface (FES) of glycyl-phenylalanyl-alanine (GFA) tripeptide was explored by molecular dynamics (MD) simulations in combination with high-level correlated ab initio quantum chemical calculations and metadynamics. Both the MD and metadynamics employed the tight-binding DFT-D method instead of the AMBER force field, which yielded inaccurate results. We classified the minima localised in the FESs as follows: a) the backbone-conformational arrangement; and b) the existence of a COOH...OC intramolecular H-bond (families CO(2)H(free) and CO(2)H(bonded)). Comparison with experimental results showed that the most stable minima in the FES correspond to the experimentally observed structures. Remarkably, however, we did not observe experimentally the CO(2)H(bonded) family (also predicted by metadynamics), although its stability is comparable to that of the CO(2)H(free) structures. This fact was explained by the former's short excited-state lifetime. We also carried out ab initio calculations using DFT-D and the M06-2X functional. The importance of the dispersion energy in stabilising peptide conformers is well reflected by our pioneer analysis using the DFT-SAPT method to explore the nature of the backbone/side-chain interactions.
Aromatic ring-peptide bond interactions (modeled as benzene and formamide, N-methylformamide and N-methylacetamide) are studied by means of advanced computational chemistry methods: second-order Moller-Plesset (MP2), coupled-cluster single and double excitation model [CCSD(T)], and density functional theory with dispersion (DFT-D). The geometrical preferences of these interactions as well as their interaction energy content, in both parallel and T-shaped arrangements, are investigated. The stabilization energy reaches a value of over 5 kcal/mol for the N-methylformamide-benzene complex at the CCSD(T)/complete basis set (CBS) level. Decomposition of interaction energy by the DFT-symmetry-adapted perturbation treatment (SAPT) technique shows that the parallel and T-shaped arrangements, although similar in their total interaction energies, differ significantly in the proportion of electrostatic and dispersion terms.
